1. Field of the Invention
The present invention relates to a machining center in which a tool change device is provided above a mobile column.
2. Description of the Related Arts
Japanese Patent Laid-Open Publication No. Hei 1-149232 published on Oct. 16, 1989 and Japanese Patent Laid-Open Publication No. Hei 7-30260 published on Jul. 12, 1995 disclose conventionally known machining centers of this type. In the machining center of the former, a tool magazine is disposed on a fixed frame provided above a mobile column and a tool change arm is provided on top of the mobile column. In the tool magazine, tools are held horizontally and are indexed within a horizontal plane by a chain. A tool indexed to a standby position is lowered to a tool delivery position by conveyance means and is exchanged with a tool on the main spindle by the tool change arm. In the machining center of the latter, tools are held horizontally in a tool magazine above a column. A tool is indexed to a standby position within a vertical plane by means of a revolving magazine plate, lowered from the standby position to a tool delivery position by conveyance means and exchanged with a tool on the main spindle by a tool change arm provided on a fixed frame.
In the former machining center, tools are lowered to the bottom of the tool magazine by conveyance means, so that the overall height of the machining center increases due to the upward and downward strokes of the conveyance means. Furthermore, since the tools are rotated by a chain, considerable force is needed to index them, resulting in the problem that a large magazine drive motor is required. Moreover, since the tool change arm is provided above the mobile column, the overall weight of the mobile column is increased, impeding high-speed movement. In the latter machining center, tools are conveyed upwards and downwards and the tool indexing direction is set within a vertical plane, thereby further increasing the overall height of the device. Another disadvantage is that, as in the chain indexing system, a large magazine drive motor is required since indexing is performed by a magazine plate.
Furthermore, in a conventional machining center, a fixed frame supported by four columns on the left and right at the front and rear is provided above a mobile column and a tool magazine is disposed on this fixed frame (support base). A tool indexed by the tool magazine is lowered to a tool delivery position below and is exchanged with a tool on the main spindle provided to the mobile column.
In this conventional machining center, the fixed frame which supports the tool magazine is itself supported by the four columns at the front, rear, left and right, so that the front left and front right columns are inevitably positioned at the sides or front of the machining region, hindering operations performed in the machining region such as, for instance, replacement or inspection of the workpieces, resulting in the problem that the machining region is restricted.
Furthermore, since a tool in the index position in the tool magazine is moved downwards in a horizontal attitude and moved to the tool delivery position for the tool change, the area below the indexed position of the tool magazine constitutes a large opening directed towards the machining space, resulting in the problem that machining chips and so on can easily enter the indexed position and cause malfunctions. In order to solve this problem, an open-close door and a cleaning device must be added, consequently raising costs.